Casting slab for shadow mask, heat treatment method thereof and material for shadow mask.
The present invention relates to a casting slab for shadow mask made of Nixe2x80x94Fe based alloy which exhibits an excellent streak suppression effect at the time of etching, the heat treatment method thereof and material for shadow mask, and more particularly to a casting slab for shadow mask made of Nixe2x80x94Fe based alloy which is suitably used as a shadow mask of a color-television cathode ray tube or a computer-display cathode ray tube, the heat treatment method thereof and material for shadow mask.
A Nixe2x80x94Fe alloy (particularly, a Fe-36%Ni alloy) which is known as an Invar alloy and is used as shadow mask material of a large-sized color-television cathode ray tube or a high-definition cathode ray tube for computer display suffers from a drawback that a stripe pattern called xe2x80x9cstreaksxe2x80x9d appears in the direction parallel to the rolling direction when the perforation by etching is performed. The segregation of Nixe2x80x94Fe which is present in a material sheet served for etching is considered to be a main cause of the generation of the streaks. This segregation is a phenomenon that the solidification segregation which is generated at the time of performing the continuous casting or the ordinary ingot casting with respect to material still remains in a final product sheet even after passing through steps which is a combination of hot working, cold working, annealing and the like after performed by the above-mentioned casting. The segregation at the time of solidification is generated in the state that it is extended in the rolling direction of the coil at the preliminary step and hence, when the product sheet is etched, they become apparent as etching irregularities in stripes parallel to the rolling direction.
Conventionally, there have been proposed several techniques for suppressing the generation of streaks at the time of etching. For example, Japanese Patent No. 2130577 (Japanese Patent Publication 78270/1995) discloses a method which suppresses the generation of streaks by applying the heat treatment above a fixed temperature and a fixed time with respect to a continuous casting slab whose solidification structure is controlled.
Further, Japanese Patent No. 2000062 (Japanese Patent Publication 78270/1995) discloses a method which applies annealing to the casting slab at a high temperature for a long time in the same manner.
Still further, Japanese Patent No. 1950743 (Japanese Patent Publication 68128/1994) discloses a method which suppresses the generation of streaks by performing heat treatment of the slab at a high temperature for a long time exceeding conditions which satisfy the relationship between a fixed temperature and a fixed time irrespective of the difference between the continuous casting and the ordinary ingot casting.
The basic principle of these prior art mainly aims at the prevention of etching irregularities which are present in the inside of the slab due to the heat, treatment at a high temperature for a long time by homogenizing the segregation of Ni, C, Si, Mn, Cr and the like by making use of the thermal diffusion.
Although Japanese Patent no. 2130577 Publication also refers to the solidification structure, what this reference implies resides in the prevention of the influence which the crystalline orientation of the solidification structure affects the crystalline orientation in a product sheet and etching irregularities derived from such crystalline orientation.
However, although in the prior art, the annealing which enables the manufacture of product sheets having sufficient characteristics as shadow masks for color-television cathode ray tubes and computer-display cathode ray tubes at that time has been possible, due to the severe mask etching conditions which become necessary along with the demand for the large-sizing and the high-definition of the computer-display cathode ray tubes in particular these days, the segregation reduction level which can be achieved with the prior art now becomes insufficient to suppress the streaks generated at the time of mask etching and hence, a further reduction of the segregation is requested.
To satisfy such a request, it is an object of the present invention to provide a sophisticated segregation reduction technique, that is, a streak reduction technique which can not be achieved by the prior art by finding out the relationship between the casting structure and the segregation at the time of solidification and the heat treatment condition corresponding to such segregation state which have not been taken into consideration in the prior art.
A first aspect of a casting slab for shadow mask of the present invention is directed to a casting slab for preparing a shadow mask which comprises an Nixe2x80x94Fe alloy containing 30 to 45% of Ni, wherein the casting slab has a cast structure comprising a columnar crystal and/or a chill crystal in an amount of 99% or more and exhibits the excellent quality with respect to streaks.
Further, a second aspect of the casting slab of the present invention resides in that the casting slab does not contain equiaxed crystal.
A third aspect of the casting slab of the present invention resides in that the casting slab is obtained by using a continuous casting method in which casting operation is carried out with no electromagnetic stirring and with maintaining a melt temperature of the non-solidificated part in the slab to a temperature equal to or above the liquidus thereof.
A heat treatment method of a casting slab for shadow mask of the present invention is characterized in that the above-mentioned casting slab is subjected to a heat treatment at a given temperature and for a given time such that the K value becomes 150 xcexcm or more.
Shadow mask material of the present invention is characterized in that the material is manufactured through steps including hot rolling, cold rolling and annealing using the above-mentioned casting slab.